A precondition for the electrostatic powder application of enamel frits with good powder adhesion is the use of a high electric surface resistance of the powder as well as good powder fluidity during the application of the enamel particles on the metal workpiece. To regulate the necessary surface resistance, enamel frits according to DE-PS 2 015 072 are coated with special insulating substances capable of reacting with Si--OH groups of the frit particles. The encapsulant is preferably added during the grinding process.
The fluidity of the powder necessary for the continuous powder application in industrial plants is substantially dependent on the composition of the frits used, their particle size as well as the type and concentration of the encapsulant. For determining the powder fluidity it was proposed to entrain a specific quantity of powder in a defined air stream and after setting up a fluidized bed of constant depth, to weight out, after a certain time, the powder quantity flowing out through a side opening. The weight of the powder flowing out is a measure for the fluidity of the powder (Lambert, Email Metal, 24, 1975, 19 et seq.).
When coating geometrically difficult shaped parts, for example in hollow spaces, insufficient powder deposition takes place, particularly in the corners in areas of low density of the lines of electric flux.
The same applies when edges are impressed onto or flanges are welded onto flat parts. The uneven powder application is particularly noticeable in color variations in these critical areas of low density of the lines of electric flux after firing the powder. In unfavorable cases, a too low enamel layer density results in the known burn off because of incomplete oxide dissolution in the glass.
An increase in the charging agent concentration does indeed generally improve the powder penetration and reduce the described defects, but simultaneously reduces the fluidity of the powder.